1. Field of the Invention
The present invention relates generally to image processing apparatus and in particular to image processing apparatus capable of preventing pseudo contour.
2. Description of the Related Art
A conventionally known image processing apparatus converts an input signal representing the density level of each pixel with a predetermined number of tones, into a signal with a number of tones smaller than the predetermined number of tones. Furthermore, an error diffusion method is also known as a technique used to generally reproduce the density level of an input signal if a tone is reduced.
FIG. 27 is a block diagram showing a configuration of an image processing apparatus employing an error diffusion method, reducing the tone of an input value for output. Herein there will be described an example with two types of dots employed to ternarize an input image. More specifically, a dot 2 corresponding to a dark dot, a dot 1 corresponding to a light dot and no dot (a dot 0) for a total of three states are used to reproduce an image.
With reference to the figure, this apparatus receives as an input value a density level of a single pixel represented by a numerical value ranging from 0 to 1 (in a multi-value), and outputs dot 0, 1 or 2.
Herein, control determines in which range the input value falls and in each range it is binarized. Herein, there are provided a range of 0 to 0.5 and a range of 0.5 to 1 for a total of two ranges. More specifically, if an input value falls within the range of 0 to 0.5 then one of dots 0 and 1 is output and if an input value falls within the range of 0.5 to 1 then one of dots 1 and 2 is output. Thus an image is reproduced.
More specifically, with reference to FIG. 27, the image processing apparatus is configured of subtracters 103 and 107 and a thresholding portion 105. Subtracter 103 receives a density level (an input value) and subtracts therefrom an error of a neighboring pixel (a correction value).
Thresholding portion 105 compares an output x of subtracter 103 with a predetermined threshold value (for example of 0.25 and 0.75 herein). If x<0.25 then 0 is output. If 0.25≦x<0.75 then 0.5 is output. If x≧0.75 then 1 is output. If thresholding portion 105 outputs 0 then dot 0 is output. If thresholding portion 105 outputs 0.5 then dot 1 is output. If thresholding portion 105 outputs 1 then dot 2 is output.
According to the output from thresholding portion 105, output x of subtracter 103 is subtracted by subtracter 107 to provide an error (a correction value) of the pixel of interest. The error is diffused to a neighboring pixel.
FIG. 28 represents a relationship between density of image data input and density of dot. For an input in a range of 0 to 0.5, dot 1 has a density increasing from 0 to 1. For an input in a range from 0.5 to 1, dot 1 has a density decreasing from 1 to 0 and dot 2 instead has a density increasing from 0 to 1. As such, for the entirety of the image, a proportional relationship is established between an input and an output, as indicated by {circle around (3)}.
Furthermore, the image processing apparatus may alternatively be configured as shown in FIG. 29.
With reference to FIG. 29, the image processing apparatus includes a decision portion 201 determining in which range an input value falls, a normalization portion 203 normalizing an input value to allow the input value to fall within a predetermined range, a subtracter 205 providing an error substraction process, a thresholding portion 207 referring to a predetermined threshold value (of 0.5 herein) to provide a thresholding process, an assignment portion 209 referring to a result obtained from decision portion 201 and a result of the thresholding process to output one of dots 0, 1 and 2, and a subtracter 211 calculating an error. Note that herein to simplify the description an input value is 0 to 1 and it is ternarized, as shown in FIGS. 27 and 28, although in decision portion 201 the number of ranges to be determined may further be increased to provide more than three values to be used for processing an image.
Reference will now be made to FIG. 30 to describe an operation of the FIG. 29 apparatus. As shown in FIG. 30 (1), an input value falls within a range of 0 to 1. Herein, if an input value is no less than 0 and less than 0.5 then decision portion 201 determines that the input value falls within a range a. If an input is no less than 0.5 and no more than 1 then decision portion 201 determines that it falls within a range b. If an input value falls within range a then the input value is normalized to fall within a range of 0 to 1, as shown in FIG. 30 (2). In contrast, if an input value falls within range b then the input value is normalized to fall within a range of 0 to 1, as shown in FIG. 30 (3). For either one of ranges a and b, thresholding portion 207 provides a thresholding process with a threshold value of 0.5. Then, if the input value in range a that is normalized is no more than 0.5 then no dot (dot 0) is output and if it exceeds 0.5 then dot 1 is output.
In contrast, if the input value in range b that is normalized is no more than 0.5 then dot 1 is output and if it exceeds 0.5 then dot 2 is output. Thus, in FIG. 29 there can be provided a process similar to that as shown in FIGS. 27 and 28.
Furthermore, the Applicant of the present application also suggests in Japanese Patent Application No. 11-237492 a threshold diffusion method diffusing a difference between a threshold value and an output value to a threshold value used to process a neighboring pixel.
FIG. 31 is a block diagram showing a configuration of an image processing apparatus employing the threshold diffusion method.
With reference to the figure, the image processing apparatus includes an input portion 901 inputting a pixel value of image data to be processed, a thresholding portion 903 thresholding a pixel value, an output portion 905 outputting a result of the thresholding process, an initial threshold generation portion 913 outputting an initial threshold value Th (x), a subtracter 915 subtracting a correction value for a threshold value from a value received from initial threshold generation portion 913, an inversion portion 907 inverting a result of the thresholding process, a subtracter 909 subtracting an output of subtracter 915 from an output of inversion portion 907 to output a correction value for a threshold value, and a β multiplication portion 911 multiplying an output of subtracter 909 by a coefficient β.
If the threshold diffusion method is applied to an image processing apparatus a correction value for a threshold value is diffused to a threshold value used to process a neighboring pixel.
Such an image processing apparatus as described above reducing a tone for output can binarize a tone of a multi-tone image (or represent an image with two states i.e., by outputting a dot and not outputting a dot) and can also apply multiple threshold values to provide a tone in more than two values (or represent an image with more than two states, such as dots of different sizes).
A digital half toning technique of a feedback system employing the above mentioned error diffusion method, threshold diffusion method and the like, has a disadvantage that a pseudo contour is created at a portion at which a dot switches, as shown in FIG. 32. A pseudo contour means a phenomenon that only a specific dot (a gray dot 1 in FIG. 32) is continuously produced more than required, as shown in FIG. 32 at the right-hand, enlarged view.
Conventionally, there has not been found a cause of such a pseudo contour and to accommodate it disturbance is exclusively applied to render the pseudo contour less noticeable.